1. Field of the Invention
The present invention generally relates to a time base corrector for video signal, and more particularly, to a time base corrector capable of converting an input video signal of lower frequency than that of a normal video signal into an output video signal of the normal frequency.
2. Description of the Prior Art
In a variable speed reproduction mode, such as a slow or quick mode, of a video signal reproducing apparatus, such as an LD, an image signal is reproduced in such a manner that track jump is performed so as to output an identical frame twice, or the reading skips over an intermediate frame. However, the above variable speed reproduction has a disadvantage in that a periodical abnormality of voice occurs because identical voice signals are successively output twice or an intermediate part of voice is skipped as in the case of the image signal.
A method using a field memory is known which is intended to eliminate the above disadvantage. This method uses a video memory having a storage capacity of at least one field. A variable speed video signal reproduced from a laser disk (LD) is written into the video memory in synchronism with a write clock corresponding to the frequency of the reproduced video signal, while the video signal written in the video memory is read out therefrom in synchronism with a read clock having a frequency of normal video signal. The structure of a memory unit of such a reproduction apparatus is illustrated in FIG. 1.
As shown in FIG. 1, the memory unit includes an A/D converter 101, a write reference signal generator 102, a field memory 110, a synchronizing signal adding circuit 104, a color phase processing circuit 105, a D/A converter 106, and a read reference signal generator 107. An input video signal has a frequency of a few percent higher or lower than the frequency of the normal video signal. This input video signal is converted to an analog video signal by the A/D converter 101, and is stored in the field memory 110. The field memory 110 has a storage capacity of at least one field. Image data is written into the field memory 110 on the basis of a write clock CK1 and a write reset signal WR. The write clock CK1 is generated by the write reference signal generator 102 and corresponds to the frequency of the input video signal. The written image data is read out from the field memory 110 in accordance with a read clock CK2 generated by the read reference signal generator 107 and a read reset signal RR. The read clock CK2 has a fixed frequency (14.3 MHz) of the normal video signal. The frequency of the read reset signal RR is determined in correspondence with the fixed frequency of the normal video signal. Hence, the video signal can be correctly reproduced.
A synchronizing signal is added to the signal read from the field memory 110 by the synchronizing signal adding circuit 104 and then subjected to a color phase processing by the color phase processing circuit 105. Then, the video signal with the synchronizing signal added thereto is converted into an analog video signal by the D/A converter 106.
In such a variable speed reproduction, there is a phenomenon called "passing". This is due to the fact that the frequency of the write clock of the video memory and the frequency of the read clock thereof differs from each other. When the frequency of the write clock is higher than that of the read clock, the time relation between the write clock and the read clock is reversed, and the write timing periodically passes (outruns) the reading timing. On the other hand, when the frequency of the write clock is lower than that of the read clock, the read timing periodically passes the write timing. When the frequency of the write clock is lower than that of the read clock, the following problem occurs in an image formed by the output video signal.
In FIG. 2, field images A-E are shown. Since the frequency of the write clock is lower than that of the read clock, the read clock passes the write clock. If the read timing passes the write timing at a position indicated by a broken line shown in FIG. 2, there is no problem on the field image A because the write operation into the video memory has already completed. However, the passing takes place while the field image B is being written into the field memory, i.e., the timing of the broken line, and from then on, the read operation into the video memory precedes the write operation from the video memory. As a result, with respect to the field image B, the stored data in the video memory is read out before all data of the field image B is completely written into the video memory. Therefore, though the data read out from the video memory before passing (timing of the broken line) is the data of the field image B, the data read out from the video memory after the passing is the data of the field image A, i.e., the field image A is read again. As a result, the lower part of the field image B is replaced by the field image A, as shown in FIG. 2.
As described above, when passing occurs, upper and lower parts of the field image become images of different fields. Actually, it is impossible to neglect the above problem if video image has a large or quick motion. For example, in a case of an image of an object moving from left to right at a relatively high speed, contents of the successive field images are considerably different. Hence, as shown in FIG. 3, image of the object cannot be correctly reproduced.
In order to overcome the above problem, it may be possible to inhibit writing of a field image into the field memory when passing occurs, as shown in FIG. 4. However, since the field image B is not written into the field memory in this case, the field image A is repeatedly output for three times.